1. Field of the Invention
This invention relates to a mold for use in rotationally blow-forming glass articles such as glass cups and electric bulbs.
This invention provides a mold having a very long life which can produce well-finished glass articles.
2. Description of the Prior Art
The so-called rotational blow-molding method is generally used for molding glass articles such as glass table ware, electric bulbs and chemical appliances. According to this method, a high-temperature molten and softened glass is blown in a mold by air and the glass or mold is rotated while contacting the glass with the inner face of the mold to thereby obtain an article of a desired form.
High temperature glass to be molded according to this molding process is generally called a glass parison, and it is generally heated at a temperature of about 1,000.degree.C.
The time required for molding one glass article is only about 5 seconds, but since the inner surface of the mold is in frictional contact with the glass parison blown under a pressure of about 0.5 Kg/cm.sup.2 during this molding period, a lining should be formed on the inner surface of the mold so that it can resist this frictional contact sufficiently. Further, the inner surface of the mold is wetted with a suitable amount of water so that molded glass can easily be parted from the inner surface of the mold. So-called cork graphite paste molds have heretofore been used generally in this molding process. Molds of this type are prepared by baking cork powder onto the inner surface of a metal mold by using a binder such as linseed oil, and the molds are thus lined with graphitized cork layers, or cork graphite layers.
However, since this cork graphite layer does not have a sufficiently lower coefficient of friction when contacted by the above-mentioned high-temperature glass parison, if the mold is used for about 8 hours, the cork graphite layer wears away and is not usable any more. This short life is one of the shortcomings of the cork graphite past mold. In addition, since it is very difficult to form a uniform cork graphite layer as a mold lining, deviation is readily caused in the quality of glass articles formed by this method and hence, articles having well-finished surfaces cannot be obtained with good consistency.
For overcoming such defects, U.S. Pat. No. 3,254,981 proposes a mold in which the inner face of the mold is lined with a porous metal as a material having a lower coefficient of friction with respect to a glass parison than graphitized cork. According to this proposal, since the glass parison is molded in direct contact with the metal lining, it is abruptly cooled and it may be broken or fine cracks may be formed during the molding step. Therefore, this method cannot be practically carried out.
Further, British Pat. No. 1,084,816 proposes an improved cork graphite paste mold. However, since cork graphite is used as a lining material, a conspicuous improvement in the length of the life of this mold cannot be expected.